Since the discovery, in 1986, of high temperature superconductors of the perovskite family, considerable effort has been directed to the development of methods of forming highly aligned thin films of these superconductors on substrates. As the technology develops from research to commercialization of systems incorporating high temperature superconductors, a need has arisen for deposition systems suitable for commercial-scale manufacture. Such systems should be suitable for large-area deposition and should produce uniform films, both on a single substrate and between substrates. The films produced should be smooth and highly oriented, with a high critical temperature, Tc, and high critical current density, jc.
One conventional method of forming superconducting films includes co-evaporation of metals, such as yttrium, barium, and copper, followed by oxidation of the deposited metals. This method can be termed “reactive co-evaporation”. The composition of the vapor in the co-evaporation chamber can be monitored or controlled using quartz-crystal monitors and a feedback device. The substrates upon which the film is to be formed are held in a rotating carousel and spaced from thermal boats that contain the material to be deposited. The substrates rotate between a deposition zone where they are exposed to mixed oxide vapors and an oxidation zone where the film is oxidized to form a superconductive oxide. The vapor pressure in the oxidation and deposition zones may differ by several orders of magnitude with very low pressures in the deposition zone. Typically, a layer with less than one unit cell thickness is formed during each rotation. The rotation allows for fast mixing of species on an atomic scale to produce a desired compound.
This method has several disadvantages. First, the thermal boats for the different materials must be close together to obtain a homogenous vapor. This limits the size of the thermal boats. Second, because the quartz-crystal monitors are not species specific, it is necessary to isolate the vapors seen by each monitor. This requires that the monitoring be done in the vicinity of the boats, rather than near the substrate. Third, the method does not allow for rapid cooling or convenient loading and unloading of the substrates. This decreases system throughput.